Convergent Evolution of a Eusuchian-Type Secondary Palate within Shartegosuchidae

2018·7 MB·English

by  Kathleen N. Dollman

A M ERIC AN MUSEUM NOVITATES
Number 3901, 23 pp. June 18, 2018
Convergent Evolution of a Eusuchian-Type Secondary
Palate within Shartegosuchidae
KATHLEEN N. DOLLMAN,1 JAMES M. CLARK,2 MARK A. NORELL,3 XU XING,4
AND JONAH N. CHOINIERE1
ABSTRACT
Shartegosuchids are a poorly known, early-branching group of Asian and North American
crocodylomorphs. Shartegosuchids have been hypothesized to have eusuchian-type secondary
palates, but a paucity of described material makes assessing this difficult. Our fieldwork in
western Mongolia recovered cranial material of a new Shartegosuchus specimen from the Ulan
Malgait Formation, which we CT-scanned and digitally reconstructed to investigate its palatal
morphology. We then incorporated this new anatomical information into a revised phyloge-
netic dataset to assess its affinities. Our study confirms that Shartegosuchus has a posteriorly
placed choana that is fully enclosed by the pterygoids, but differs from Eusuchia in possessing
a secondary palatal fenestra and reduced palatine bones. Shartegosuchus, together with Adzho-
suchus, Fruitachampsa, and Nominosuchus, forms the monophyletic group Shartegosuchidae.
Shartegosuchidae is nested within a larger clade Shartegosuchoidea, and this clade is an earlier-
diverging lineage than Eusuchia, showing that a eusuchian-type secondary palate evolved mul-
tiple times in crocodylomorphs, including very early in the group’s evolutionary history. The
co-occurrence of Nominosuchus in the Ulan Malgait Formation and the Shishigou Formation
allows us to assign an early Oxfordian age to Shartegosuchus. The independent evolution of a
eusuchian-type secondary palate in an oreinorostral group suggests that the link between platy-
rostry and a closed secondary palate has been overstated.
Keywords: Crocodylomorpha, palatal fenestra, pterygoid, Ulan Malgait Formation,
Mongolia
1 Evolutionary Studies Institute and School of Geosciences, University of the Witwatersrand, Johannesburg.
2 The George Washington University, Washington, D.C.
3 Division of Paleontology, American Museum of Natural History.
4 The Institute of Vertebrate Paleontology and Paleoanthropology, Beijing.
Copyright © American Museum of Natural History 2018 ISSN 0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3901
INTRODUCTION
The secondary palate of extant crocodylians and their eusuchian relatives is a derived ana-
tomical feature that may serve as an adaptation to aquatic habits, and whose functionality has
been a subject of speculation for nearly 150 years. Macroevolutionary hypotheses for the evolu-
tion of this derived palatal construction generally involve a series of intermediate conditions
(Langston, 1973). The early diverging members of Crocodylomorpha, “protosuchians,” are
often described as having a “protosuchian” palate (Langston, 1973; Buffetaut, 1982), which is
open and has the choana positioned anteriorly between the palatine and a maxillary secondary
palate. More crownward extinct taxa are considered to have a “mesosuchian” palate (hereinaf-
ter: “palatine secondary palate”), formed from the maxilla and palatines, with a posteriorly
positioned choana bordered anteriorly by the palatines and posteriorly by the pterygoid. Finally,
eusuchians have a fully closed, bony secondary palate with the choana positioned posteriorly
on the palate and enclosed entirely by the pterygoids (hereinafter: “pterygoid secondary pal-
ate”) (Huxley, 1875; Langston, 1973).
Several recent studies have shown that the construction of the crocodylomorph secondary
palate and position of the choana have complex evolutionary histories, with multiple conver-
gences and reversal events and with many different palatal structure formations (Turner and
Buckley, 2008; Pritchard et al., 2013; Wilberg, 2015). A palatine secondary palate is present in
many groups, including some shartegosuchids (Nominosuchus and Fruitachampsa), thalattosu-
chians, and goniopholidids, although some members of the latter taxon have an open nasopha-
ryngeal duct, e.g., Eutretauranosuchus (Pritchard et al., 2013). A pterygoid secondary palate is
present in Eusuchia, Mahajangasuchus, and reportedly in the shartegosuchids Shartegosuchus
and Adzhosuchus (Efimov et al., 2000). Thalattosuchia is generally considered to be a part of a
pelagic clade deeply nested within Crocodyliformes, e.g., Clark (1994). However, there is some
evidence that supports Thalattosuchia as sister to Crocodyliformes, which would imply that
the palatine secondary palate arose in thalattosuchians and was lost in early-branching croco-
dyliforms (Benton and Clark, 1988; Jouve, 2009; Pol and Gasparini, 2009; Wilberg, 2015).
Goniopholidids are nested among neosuchian taxa and exhibit significant diversity in their
palatal anatomy: some forms possess a complete palatine secondary palate; others possess an
open nasopharyngeal duct separating the palatines medially (Tykoski et al., 2002; Pritchard et
al., 2013). The phylogenetic position of Mahajangasuchus is uncertain, but under all analyses
it is recovered far from Eusuchia (Turner and Buckley, 2008; Sereno and Larsson, 2009; Turner
and Sertich, 2010; Pol and Powell, 2011). This implies that the pterygoid secondary palate of
Mahajangasuchus is not homologous with that of eusuchians, which is supported by many dif-
ferences in its anatomy (e.g., the expanded pterygoid septum forms the anterior border of the
choana, excluding the palatines).
Early hypotheses for the posterior position of the choana and the closed secondary palate
in eusuchians focused on the role it plays in separating the nasal passage from the oral cavity
(Huxley, 1875; Langston, 1973). More recently, the development of a closed eusuchian-type
palate was hypothesized as a means of strengthening the platyrostral skull against strains placed
on it during feeding (Busbey and Gow, 1984; Rayfield and Milner, 2008), with the posteriorly
2018 DOLLMAN ET AL.: CONVERGENT EVOLUTION IN SHARTEGOSUCHIDAE PALATE 3
placed choana being a by-product of this functional constraint. The observation of a closed
palate and a posteriorly placed choana in Shartegosuchus (Efimov et al., 2000), a crocodylo-
morph with an oreinorostral snout, calls into question the strength of the functional relation-
ship of a closed palate and a posteriorly placed choana with a platyrostral skull.
Shartegosuchids are a poorly known clade of small-bodied crocodyliforms from Asia and
North America. Some shartegosuchids possess an unusual, well-developed palate that is thick
and heavily sculptured, such as Adzhosuchus, Shartegosuchus, and Fruitachampsa (Efimov et
al., 2000; Clark, 2011). An incomplete, “mesosuchian” palatine secondary palate is present in
Nominosuchus and Fruitachampsa, but interestingly, the later-branching taxa Shartegosuchus
and Adzhosuchus have been described as possessing a pterygoid secondary palate. Additionally,
some forms (e.g., Fruitachampsa and Shartegosuchus) also possess two palatal openings: an
anterior palatal fenestra and a posterior opening likely homologous to the choana. Thus,
shartegosuchids present a rare instance in which the evolutionary transformation sequence
between the two palatal constructions is seen within a lineage (Efimov, 1996; Efimov et al.,
2000; Clark, 2011).
The 2010 Mongolian-American Expedition to the early Late Jurassic exposures in the west-
ern Gobi recovered several well-preserved specimens of basal crocodylomorphs, including new
specimens of Shartegosuchus asperopalatum from the Ulan Malgait Formation. Here we present
the results of CT-scanning and digital reconstruction of a well-preserved snout of Shartegosu-
chus (IGM 200/50), and discuss its implications for palatal evolution within
Crocodylomorpha.
METHODOLOGY
A partial skull of Shartegosuchus asperopalatum (IGM 200/50) was CT-scanned at the
American Museum of Natural History Microscopy and Imaging Facility. Individual skull bones,
including all palatal elements, were segmented into separate regions using the region-growing
tool in VG Studio Max 3.0. The scan was compared to previous descriptions of the palates of
other crocodylomorphs (Iordansky, 1973; Busbey and Gow, 1984; Efimov et al., 2000; Gow,
2000; Tykoski et al., 2002; Turner and Buckley, 2008; Clark, 2011; Pritchard et al., 2013).
The relationship of Shartegosuchus to other crocodylomorphs was tested in a matrix
developed by Pol and Gasparini (2009) and modified by Clark (2011). The data matrix was
compiled and edited using Mesquite v3.10 (Maddison and Maddison, 2018). One new char-
acter was added concerning the position of the choana relative to the suborbital fenestra.
Characters 200 and 258 were amended to introduce more specific information concerning
the sculpturing of palatal elements, and the size of the palatine and its participation in the
margins of the suborbital fenestra.
The taxonomic sample used in Clark (2011) was maintained, which included the shartego-
suchid genera Shartegosuchus, Adzhosuchus, Fruitachampsa, and Nominosuchus. Gracilisuchus
was specified as the outgroup. New anatomical information described for IGM 200/50 was
incorporated into a coding of Shartegosuchus asperapalatum (which included PIN 4171/2).
Twenty-three character codings for Shartegosuchus that were previously unknown were scored,
4 AMERICAN MUSEUM NOVITATES NO. 3901
and four additional character scores were amended (see online supplementary information at
https://doi.org/10.5531/sd.sp.31).
The data matrix was analyzed in TNT version 1.5 to heuristically search for shortest length
topologies (Goloboff and Catalano, 2016). Memory was set to 10,000 trees and 100 MB of
RAM. The data matrix was analyzed with the following parameters: tree bisection and recon-
nection (TBR) as a heuristic search strategy; 1000 Wagner builds with a random seed of 1; keep
two trees per replication; and replace existing trees when more optimal trees were discovered.
An additional round of branch swapping on shortest-length topologies was then performed
using the trees in memory and holding up to 10,000 trees of shortest length. Support for the
strict consensus tree topology was assessed using Bremer support, jacknife, and bootstrap.
Bremer support was calculated in TNT by a round of TBR swapping on the optimal topologies,
saving trees up to 10 steps longer than the MPT’s and stopping when the tree buffer contained
10,000 topologies of any length. Jacknife and bootstrap support measures were calculated in
TNT using GC frequencies. Jacknife analysis used 36 removal probability and 100 replicates.
Bootstrap analysis used a standard resampling strategy (with replacement) and 100 replicates.
An additional analysis constraining Thalattosuchia as the sister-taxon of Crocodyliformes was
performed to evaluate how the position of Thalattosuchia affects inferences of palate evolution
within Crocodyliformes, given the new data collected on shartegosuchids. The constrained
analysis was completed by defining tree constraints in TNT and repeating the analysis using
the same search parameters outlined above.
SYSTEMATIC PALEONTOLOGY
Crocodyliformes Hay 1930 (sensu Clark, in Benton and Clark, 1988)
Shartegosuchoidea Efimov, 1988
Shartegosuchidae Efimov, 1988
Revised Familial Diagnosis (* denotes revised diagnostic features): *pterygoid, palatine
and maxilla are sculptured with concentrated, deep, irregular-shaped pitting; posterior maxil-
lary teeth and postcaniniform dentary teeth with flat, horizontal cusp and vertical crenulations
extending proximally; mandibular fenestra absent, *dentary anteriorly edentulous.
Shartegosuchus asperopalatum Efimov, 1988
Holotype: PIN 4174/2 a dorsoventrally crushed skull and mandible of a juvenile
individual.
Type Locality: Ulan Malgait Formation, Shar Teg, Mongolia.
Referred Material: IGM 200/50, an anterior portion of the skull including rostrum and
palatal and dentary bones recovered from Ulan Malgait Formation, Shar Teg, Mongolia.
Revised Genus and Species Diagnosis: *Palatines small and enclose anterior palatal fenestra;
*mediolaterally narrow and anteroposteriorly elongate anterior palatal fenestra; *anterior palatal
2018 DOLLMAN ET AL.: CONVERGENT EVOLUTION IN SHARTEGOSUCHIDAE PALATE 5
A Russia B
Mongolia
Shartegoschus
IGM 200/50 Locality
China
FIGURE 1. A. Map of Mongolia with marker indicating the locality of Shartegosuchus asperapalatum (IGM
200/50). B. Photograph of locality of Shartegosuchus asperapalatum (IGM 200/50) with arrow indicating the
position the specimen was recovered from.
fenestra begins at level of midpoint of suborbital fenestra and ends beyond anterior margin of sub-
orbital fenestrae; *choana positioned posteriorly on the palate and enclosed by pterygoids.
The name Shartegosuchoidea is proposed for the clade Shartegosuchidae + Shantungosu-
chus, Sichuanosuchus, and Zosuchus, and we define it here as the least-inclusive clade including
Shartegosuchus, Shantungosuchus, Zosuchus, and Sichuanosuchus. The definition of Shartego-
suchidae is revised as the most inclusive clade that includes Shartegosuchus and Nominosuchus
but not Zosuchus.
RESULTS
Stratigraphy
The Shartegosuchus specimen (IGM 200/50) was discovered in a gray, medium-coarse-
grained sandstone lag deposit at the Bor Ukhaa locality within the reddish mudrock strata of the
lower part of the Ulan Malgait Formation exposed at Shar Teg in Gobi Altai Aimag (fig. 1A).
The Ulan Malgait beds have yielded fossil remains of fish, turtles, crocodyliforms, dino-
saurs, a tritylodontid, molluscs, and ostracods (Gubin and Sinitza, 1996; Watabe et al., 2004;
Watabe et al., 2007). Remains of both Shartegosuchus and Nominosuchus have been recovered
at this locality (Efimov et al., 2000). Gubin and Sinitza (1996) assigned the Ulan Malgait beds
to the Upper Jurassic based on the fossil fauna assemblages found at the Shar Teg locality.
The cooccurrence of the shartegosuchid Nominosuchus matutinus (Efimov et al., 2000) in the
upper part of the Shishigou Formation at Wucaiwan, China (Clark and Xu, 2009a; 2009b), and the
Ulan Malgait Formation strongly suggest these formations are coeval. Identification of Shishigou
Nominosuchus individuals is based on descriptions by Efimov (1996) of Shar Teg specimens and a
study of the original material by James Clark. Future anatomical investigation of the Nominosuchus
specimens from both localities will confirm or revise their taxonomic identifications. Radiometric
dating of tuffs at the Shishigou Formation indicate these beds are 159.7+/-0.3 and 162.2+/-0.2 Ma
(Choiniere et al., 2014), which suggests that the Shartegosuchus specimen from the Ulan Malgait
Formation is in the early Oxfordian stage near the beginning of the Late Jurassic (Gradstein, 2012).
6 AMERICAN MUSEUM NOVITATES NO. 3901
Description and Comparisons
Only the rostrum, anterior rim of the orbits, palate, and the anterior portion of the man-
dible are preserved. The bone is white, preserved in a medium-to-fine-grained yellow sand-
stone matrix. The dorsal bones of the rostrum are fragmentary and posteriorly only their
impression in matrix remains. The anterior section of the mandible is preserved in tight occlu-
sion with the cranium. The lower jaws have been broken anterior to the mandibular fenestrae,
preserving no detail of the fenestrae or of the articular region.
The antorbital fossa, described as the “maxillary depression” by Efimov et al. (2000), is a large
fossa on the lateral surface of the rostrum, anterior to the orbit (figs. 2, 4). The depression is in a
similar position as the antorbital fossa found in non-shartegosuchoid “protosuchians,” e.g., Ortho-
suchus (Nash, 1975) and Hemiprotosuchus (Bonaparte, 1971), however, it fails to open into a large
antorbital fenestra. The depression is circular in outline and the surface of the fossa is unsculp-
tured. An antorbital fossa is also described for Adzhosuchus and Nominosuchus at the confluence
of the maxilla, lacrimal, and jugal (Efimov, 1996). Similar depressions are present within gonio-
pholidids (Tykoski et al., 2002; Pritchard et al., 2013) and some pholidosaurs (Martin and Buf-
fetaut, 2012), although they are unlikely to be homologous, based on the phylogenetic distribution
of these groups in Crocodylomorpha (Turner and Buckley, 2008). The depression is absent in
Fruitachampsa (Clark, 2011). Shartegosuchids appear to lack an antorbital fenestra except for
Nominosuchus, which has a small opening. Within shartegosuchoids, Sichuanosuchus has a small
antorbital fenestra that opens from a shallow antorbital fossa (Wu et al., 1997), Zosuchus has a
small antorbital fenestra that is not surrounded by an antorbital fossa (Pol and Norell, 2004), but
unknown within Shantungosuchus (Young, 1961; Wu et al., 1994).
The palatal fenestra is positioned on the midline of the palate and enclosed entirely by the
palatines (fig. 3). The palatal fenestra is an oblong slit with the minor axis oriented mediolater-
ally and the major axis oriented anteroposteriorly. Posteriorly, the palatal fenestra begins at the
level of the midpoint between the suborbital fenestrae, and it extends anteriorly beyond the
anterior margin of the suborbital fenestrae. In contrast, the palatal fenestra of Fruitachampsa
is positioned anterior to the level of the suborbital fenestra and is circular, whereas the palatal
fenestra of Nominosuchus is a mediolaterally narrow slit confluent with the choanal opening
posteriorly (Efimov et al., 2000; Clark, 2011).
The suborbital fenestra is enclosed by the maxilla anteriorly and laterally, the pterygoid medi-
ally and posteriorly, and presumably the ectopterygoid posterolaterally. The suborbital fenestra is
oblong in shape, with the posterior margin approximately twice as wide as the anterior margin.
The choana is oval in outline, with a raised, unsculptured surface rimming its anterior and
lateral edges. It is enclosed entirely by the pterygoid, and positioned posteriorly on the palate
medial to the pterygoid flanges (fig. 3). The major axis is oriented along the longitudinal axis
of the skull.
Maxilla: The body of the maxilla is divided into a facial portion and a palatal process.
The facial portion of the maxilla extends dorsally and curves medially, contacting the premax-
illa anteriorly, nasal dorsomedially, and the lacrimal posterodorsomedially where it forms the
lateral wall and a portion of the dorsal roof of the rostrum (fig. 4). Posteriorly, the maxilla
2018 DOLLMAN ET AL.: CONVERGENT EVOLUTION IN SHARTEGOSUCHIDAE PALATE 7
A
pty
ect
j
pf
lac
ant fos
v
m
pm
n
10 mm
B
pal fen
d
m
pal
sp
sub fen
ch
pty ang
FIGURE 2. Digital reconstructions and line drawings of Shartegosuchus asperapalatum (IGM 200/50) in
A, dorsal view, B, ventral view of Shartegosuchus asperapalatum (IGM 200/50). Abbreviations: ang, angular;
ant fos, antorbital fossa; ch, choana; d, dentary; ect, ectopterygoid; j, jugal; lac, lacrimal; m, maxilla; n,
nasal; pal, palatine; pal fen, palatal fenestra; pf, prefrontal; pm, premaxilla; pty, pterygoid; sub fen, sub-
orbital fenestra; sp, splenial; v, vomer.
8 AMERICAN MUSEUM NOVITATES NO. 3901
pm
m
pal
pal fen
pty
ect
ch
FIGURE 3. Digital reconstructions and line drawings of Shartegosuchus asperapalatum (IGM 200/50) in ven-
tral view of the palate. Abbreviations: ch, choana; ect, ectopterygoid; m, maxilla; pal, palatine; pal fen, palatal
fenestra; pty, pterygoid.
contacts the jugal ventrally along the anterior edge of the orbit. The facial surface of the maxilla
is marked by a large antorbital fossa. The unsculptured surface of the fossa is in contrast with
the remaining rostral surface of the maxilla, which is marked with extensive pitting.
The palatal shelves of the maxillae, together with the missing portion of the premaxillae,
contact to form the anterior end of the palate (fig. 3). Posteriorly, the maxillae are not in
contact at the midline, being separated by the palatines and palatal fenestra, but there is still
a considerable contribution of this portion of the maxillary shelf to the secondary palate. In
non-shartegosuchoid “protosuchians” the palatal shelves of the maxilla are separated poste-
riorly by the choana, which is positioned anteriorly on the palate. The vomers in non-
shartegosuchoid “protosuchians” are also exposed in ventral view of the palate, a condition
not apparent within shartegosuchoids.
Unlike the palatine and pterygoid, the ventral surface of the maxillary palatal shelves is
unsculptured, except along the contact with the palatine and pterygoid where the maxilla is
sculptured with pitting and ridges. The maxilla does not appear to participate in the palatal
fenestra margins, however, there is a fracture along this region and an apparent missing portion
of the maxillary palatal shelf along the midline of the skull and another missing portion along
2018 DOLLMAN ET AL.: CONVERGENT EVOLUTION IN SHARTEGOSUCHIDAE PALATE 9
A pmant fos lac pf
n
j
ect
10 mm ang
lac
B pty ant fos m
sp d
FIGURE 4. Digital reconstructions and line drawings of Shartegosuchus asperapalatum (IGM 200/50) in
A. left lateral view, B. right lateral view of Shartegosuchus asperapalatum (IGM 200/50). Abbreviations: ang,
angular; ant fos, antorbital fossa; d, dentary; ect, ectopterygoid; j, jugal; lac, lacrimal; m, maxilla; n, nasal;
pf, prefrontal; pm, premaxilla; pty, pterygoid; sp, splenial.
the anterior edge of the palatines. In Nominosuchus and shartegosuchoids Sichuanosuchus and
Shantungosuchus, the margins of the palatal fenestra are formed by both the maxilla and pala-
tine (Wu et al., 1994; Wu et al., 1997; Efimov et al., 2000; Clark, 2011). The maxilla forms the
anterior, lateral, and the rostral half of the medial margin of the suborbital fenestra. This is
different from the condition described for non-shartegosuchoid “protosuchians,” where the
maxilla is excluded from the anteromedial edge of the suborbital fenestra by an anterolateral
maxillary process of the palatines. The maxilla has eight alveoli, the first two of which are large,
housing two large caniniform teeth. The more posterior alveoli are much smaller and gradually
decrease in diameter posteriorly. The first and second alveoli are joined and circular in cross
section, the remaining smaller alveoli are oval in cross section and are separated by bony septa.
The rostral surface of the maxilla bulges laterally to accommodate the two large caniniform
teeth. The remaining anterior surface of the maxilla that abuts the premaxilla is incomplete.
Lacrimal: The lacrimal is a small bone that contacts the jugal posteriorly, the maxilla
ventrally and anteriorly, and the prefrontal medially. The lacrimal forms the posterodorsal
surface of the antorbital fossa. The posterior edge of the dorsal surface of the lacrimal is cupped,
presumably to articulate with a corresponding facet on a palpebral.
Prefrontal: The prefrontal is a small, rectangular bone that contacts the lacrimal antero-
laterally and the pterygoid ventromedially. The prefrontal pillar of Shartegosuchus IGM 200/50
is preserved as a thin sheet of bone that extends ventromedially from the body of the prefrontal
to form a weak contact with the lateral area of the dorsal surface of the pterygoid (fig. 4). This
10 AMERICAN MUSEUM NOVITATES NO. 3901
is unlike the prefrontal pillar observed in extant crocodylians wherein the prefrontal forms a
robust contact with the dorsal surface of the palatines (Iordansky, 1973).
Jugal: Only the anterior ramus of the left jugal is preserved (fig. 4A). It curves strongly
dorsally as it extends anteriorly, so that the dorsal margin is concave. This margin forms the
ventral and anteroventral margin of the orbit. The tip of the anterior ramus of the jugal overlaps
the ventralmost extension of the lacrimal for a short distance. The lateral surface of the jugal
is incised by a shallow groove that extends anteroposteriorly along the anteroventral margin of
the orbit. Dorsal to this groove, the jugal is unsculptured, and ventral and posterior to the
groove, the jugal is ornamented with shallow pitting.
Palatine: The palatine is restricted to the medial portion of the palate as a small, crescent-
shaped element. The ventral surface of the palatine is highly sculptured with pitting, except
along the margin of the palatal fenestra, where it is lacking. The palatal sculpturing consists of
small circular or oblong shallow pits that are either linked or separated by a series of shallow
ridges. The pits closer to the midline of the palate are more circular in shape. The pits become
more oblong laterally on the palate. Also, posteriorly on the palate, the pitting becomes more
deeply incised and the ridging more pronounced.
The palatine is excluded from the margins of the suborbital fenestra by the posterior exten-
sion of the palatal shelf of the maxilla and the anterior ramus of the pterygoid. The palatines
enclose an opening, the palatal fenestra, which is positioned on the midline of the palate and
medial to the suborbital fenestra.
Pterygoid: The fused pterygoids form the posterior surface of the palate. A ridge running
along the midline of ventral palatal surface extending anteroposteriorly from posterior edge of
the palatal fenestra to the anterior edge of the choana could be interpreted as a suture between
the pterygoids. However, in CT cross section the suture between individual pterygoid bones can-
not be observed, revealing that the pterygoid is completely fused. The palatal surface of the ptery-
goid is heavily sculptured with pits and ridges, which become more pronounced posterolaterally
along the pterygoid flange (fig. 3). Pitting is also noted on the palatal surface of shartegosuchoids
Shantungosuchus but not Sichuanosuchus, and extensive on the palatal surface of shartegosuchid
Fruitachampsa. Efimov (1996) illustrates Nominosuchus as also having a sculptured palatal surface
of the pterygoid (holotype PIN 4174/4), however, study of referred specimen IVPP 14392 shows
a small amount of sculpturing of the palatal surface of the pterygoid, palatines, and maxilla.
The pterygoid has an anterior ramus that contacts the prefrontal, maxilla, palatine, and
vomer and a posterolateral ramus that forms the pterygoid flange. The anterior ramus of the
pterygoid forms the posterior portion of the medial edge of the suborbital fenestra. Anteriorly,
along the margins of the suborbital fenestra, the pterygoid contacts the maxilla in mediolater-
ally transverse suture. The medial surface of the anterior ramus of the pterygoid is concave and
sutured to the convex edge of the palatine. The anterior ramus of the pterygoid has a tall ridge
on the dorsal surface of the pterygoid that extends anteroposteriorly along the midline of the
palate over the palatal fenestra. The ridge is tallest at the contact point with the prefrontal pil-
lars (fig. 4A). The anterior tip of the dorsal ramus is incomplete, but appears to have contacted
the posterior edge of the vomer along the anterior edge of the palatal fenestra (fig. 2A). In CT